1. Field of the Invention
The present invention is directed to an apparatus for and a method of conditioning glazed polishing pads. More specifically, the present invention focuses upon an apparatus for and a method of conditioning chemical mechanical polishing pads during the polishing of workpiece surfaces.
2. Background of the Prior Art
A critical step in the formation of workpieces, for example, semiconductor wafers, is the polishing of their surfaces prior to processing. There are major, related problems associated with this operation. A first problem is the requirement of replacing the polishing pad after about 500 cycles, thus adding expense and slowing the continuous nature of this operation. A second, related problem is obtaining optimum planarization during the chemical mechanical polishing process.
The above problems are associated with the basic aim of chemical mechanical polishing, a clean uniform surface. This result, in turn, is a function of the uniformity of the polishing pad. A polishing pad must itself be uniform to insure that the polished surface of the workpiece is similarly uniform.
It has been found that hard polyurethane pads provide the best uniformity when the workpiece is a semiconductor wafer. However, as those skilled in the art are aware, a polyurethane pad is characterized by an open pore structure. As such, the pad requires a regimented conditioning cycle to remove debris that accumulates on its surface and in its pores. Thus, a system has developed wherein a workpiece, e.g. a semiconductor wafer, or a plurality of workpieces are polished by the polishing pad followed by a step of removing the debris, denoted in the art as glazing, from the polishing pad surface. Glazing is a collective term for polished off workpiece residual, polishing slurry and pad material that presses into pores of the polishing pad. Glazing of the polishing pad surface has adverse consequences which include reduced workpiece surface removal rates, reduction of thickness uniformity across the workpiece and a general increase in workpiece surface defects. These surface defects include scratches, divots and the like.
To reduce polishing pad degradation, the prior art has developed pad conditioning methods. In general, these methods involve abrading the surface of the pad material by means of a mechanical apparatus which rotates a diamond impregnated disk on the pad surface. Although this method has increased useful pad life by reducing glazing degradation, it suffers from several defects. The most obvious of these defects is the systemic destruction of the pad over time. This drawback is obvious insofar as the surface of the pad is continually diminished by this abrading step. At present, the best polishing pads, e.g. the lowest abrading polyurethane pads, yields no more than about 500 workpiece cycles per pad.
Not only does the polishing pad conditioning system of the prior art have the drawback of requiring replacement of the polishing pad after approximately 500 cycles but, in addition, because the diamond impregnated disk wears over time, so does the uniformity of the polishing pad resulting in non-uniform polished workpiece surface over the approximately 500 cycle useful life duration of the polishing pad.
In view of the revolutionary change in pad conditioning accomplished the present invention, it is not necessary to discuss prior art references which utilize systems typical of the art described above. Those systems, methods and apparatus are so far removed from the present invention as to be totally irrelevant thereto. However, the present invention utilizes a system developed by the Cryogenesis.RTM. Company. The Cryogenesis.RTM. system utilizes a pelletizer which converts liquid carbon dioxide into high density dry ice pellets. The pellet length, diameter, density and the rate at which the pellets are generated is computer controlled. The Cryogenesis.RTM. pelletizer is provided with a gun which delivers the dry ice particles at high velocity to a surface. All this is described Cryogenesis.RTM. literature and is known in the art.
U.S. Pat. No. 5,036,630 sets forth a method of polishing a semiconductor wafer utilizing wafer polishing apparatus of the type utilized in the present invention. However, the method employed in conditioning the polishing pad is typical of processes utilized in the prior art whose disabilities are discussed above.
The above remarks establish the need in the art for a new method of chemical/mechanical polishing (CMP) process which provides a more efficient and longer lasting method of pad conditioning.